More than 10% of patients undergoing a simple inguinal hernia repair have neuropathic pain at the site two years postsurgery. The presence of the persistent pain does not appear though to depend on either surgical technique or the extent of damage to the ilioinguinal nerves. These findings suggest that a genetic component contributes to the risk of developing post-surgical neuropathic pain. Transection of the peripheral axons of primary sensory neurons results in alterations in their metabolism, regenerative capacity, survival, excitability and transmitter function, and we find using microarrays, that many hundreds of genes are regulated in the dorsal root ganglion following a peripheral nerve injury (Costigan et al, 2002). A major challenge is to identify which of the injury-regulated genes establish and maintain the pain, and whether the genes contributes to an increased susceptibility to develop neuropathic pain. To address this issue we have perfomed a pilot study using whole genome expression arrays to study the transcription profiles of DRGs in two closely related strains of rats with different neuroapthic pain phenotypes. We examined rats congenic for a high or a low autotomy phenotype following peripheral nerve transection (HA and LA respectively). Of 31,000 transcripts assayed in the DRG of naive and nerve-injured HA and LA rats, we find two that show a significant differential regulation between the strains; a full length EST with no known homology and a serine protease inhibitor. The protease inhibitor is substantially upregulated in the LA (low neuropathic pain sensitivity) strain and not in the HA (high neuropathic pain sensitivity) rats. Serine proteases activate bradykinin as well as many other neuoractive proteins and have been implicated in producing neuropathic pain. We hypothesise that the serine protease inhibitor regulates the level of axonal hypersensitivity and ectopic activity after nerve injury. To study this we propose to: 1. Characterize the expression of the serine protease inhibitor within the DRGs of HA / LA rats and in C57/BI6 / DBA/2 mice, both in naive animals and after nerve injury, and 2.Characterize the serine protease inhibitor's function in vivo using conditional overexpressing transgenic animals. We will create transgenic mice that will express the serine protease inhibitor at high levels in sensory neurons or non neuronal cells following nerve injury, and predict that this should, by mimicking the expression in the LA rats, decrease pain hypersensitivity in diverse neuropathic models.